1. Field of the Invention
The present invention relates to optical filters. More specifically the invention relates to semiconductor optical filters having a substrate and a multi-layer optical thin film deposited thereon.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art
Optical detectors are used in optical space sensors and other applications. In many cases, the detector is an infrared sensor combined with an optical filter. The optical filter defines the spectral band of radiation that will be sensed by the detector. One such optical filter is known as a long wavelength infrared (LWIR) multilayer dielectric (MLD) interference filter.
LWIR MLD filters are constructed by coating a semiconductor or dielectric substrate with alternating layers of high refractive index and low refractive index optical thin film materials. By depositing high and low refractive index materials of the proper optical thickness, these interference filters can be made to reflect certain out-of-band wavelengths while transmitting in-band wavelengths. Thus by altering the thickness of each layer of the filter, a controlled amount of radiation in a narrow band of frequencies will be allowed to enter the detector.
The typical design goals of conventional long wavelength infrared (LWIR) filters are to achieve: 1) broadband rejection, i.e., the filter should have a low out-of-band transmission; 2) high in-band transmission, allowing the selected transmission bands to enter the detector with minimum attenuation; and 3) a sharp cut-off slope, that is, the transition between transmitting and nontransmitting wavelength regions should be clearly defined.
The need for broadband rejection requires an MLD filter of conventional construction with many layers. It is not uncommon for as many as 300 layers to be used in the construction of a single MLD filter. Each layer is one-quarter of the operating wavelength divided by the index of refraction. Thus with an operating wavelength of 20 microns and an index of refraction of say 2, each layer must be at least 2.5 microns thick. Three hundred such layers would be 750 microns thick. At this thickness, the interference filter may be thicker than the dielectric or semiconductor substrate. Thus, the filter must often be split between two or three substrates. This creates a packaging problem.
Also the many layers of coating limits the speed (f-number) of the optical system. This is due to the fact that the spectral width of the transmitting region of these thick interference filters is shifted for incoming radiation at large angles of incidence (i.e. low f-number). Thus in low f-number systems the transition between the transmitting and nontransmitting regions would occur at the wrong wavelength. This limitation results in a larger optical system (i.e. high f-number) which is undesirable.
These optical filters are designed to function in cryogenic environments and therefore must be durable as well as functional. In accordance with conventional design techniques, it is difficult to fabricate the filter for such environments due to stress and adhesion problems associated therewith. Hence, at present, only a few optical filter manufacturers have the capability of making these filters. Accordingly, the filters tend to be quite expensive.
Thus, there is a need in the art for a compact, durable, and cost effective filter which affords greater control of in-band and out-of-band transmission and is insensitive to the system f-number.